1. Field of the Invention
The present invention relates to an indexable type rotary milling tool and an indexable insert used therefor.
2. Description of the Related Art
Conventionally, there have been dominantly used indexable type rotary cutting tools using indexable inserts each designed in a triangular shape, a rectangular shape, a rhombus shape or the like. In these tools, the main cutting edge is of a straight cutting edge type. A feed per one tooth as a general cutting condition is equal to 0.1 to 0.5 mm/tooth for casting-cutting and 0.1 to 0.3 mm/tooth for steel-cutting although it is varied according to depth and width of cut. However, such examples begin to appear that the use of an indexable type rotary cutting tool using a button insert implements the working at a feed which is not less than about two times as large as a straight cutting edge, thereby resulting in enhancement of the working efficiency. This indexable type rotary cutting tool uses button positive type inserts including various sizes of 8 to 20 mmxcfx86. The clamping mechanisms therefor are broadly classified into two kinds, one kind being designed to fix the insert by pieces of a wedge and a supporter, and the other being designed to fix the insert by a fastening screw inserted into a pin hole disposed at the center of the insert.
Further, the button insert is slightly large in cutting resistance because of its shape-dependent characteristic, but longer in length of the cutting edge than the straight cutting edge, and further high in insert strength; thereby providing many advantages of making it possible to cut a high hardness material, and then lengthening the lifetime of the insert, etc. As an example using such a button insert is proposed tools as described in Japanese Laid-open Patent Application (Kokai) Nos. 9-225724 and 10-118901.
However, in recent high efficiency working (contour line working), such cases begin to appear that chattering-vibration is caused during a process of cutting the corner portions and the grooves, thereby finally causing the inserts to be damaged.
It is therefore an object of the present invention to suppress the chattering-vibration during the cutting process of the corner portions and the grooves to thereby realize a stable and high efficiency working (contour line working).
In order to attain the above object, according to a first aspect of the present invention, there is provided an indexable type rotary milling tool having at least one indexable insert, characterized in that the indexable insert has a main cutting edge and an outer peripheral cutting edge (9), the main cutting edge ranges from 3 to 35 degrees in cutting edge angle (xcexa) and is shaped like an arc or a substantially arc, and the outer peripheral cutting edge is shaped like a straight, an arc, or a substantially arc so as to be back-tapered with respect to the center axis of the tool.
Further, according to another aspect of the present invention, there is provided an indexable insert shaped like a substantial rectangle and having a pin hole and a relief angle, characterized in that the indexable insert is of a two-corner-used type, and shaped like a point-symmetry with respect to the center of the pin hole, each of two sides symmetrical with respect to the center of the pin hole, out of four sides forming the substantial rectangle, is shaped like a straight line, an arc or a substantially arc. Each of the other sides out of the four sides comprises an arc portion (7) shaped like an arc or a substantially arc, and a straight portion continuing from the arc portion, a line segment extending from the arc-center of the arc portion to the connecting point between the arc portion and the straight portion intersects to the straight portion at an angle (xcex1) of less than 90 degrees.
Besides, according to another aspect of the present invention, there is provided an indexable insert shaped like a triangle and having a pin hole and a relief angle, characterized in that each of three sides forming the substantial triangle comprise an arc portion shaped like an arc or a substantially arc and a straight portion continuing from the arc portion, a line segment extending from an arc-center of the arc portion to the connecting point between the arc portion and the straight portion intersects to the straight portion at an angle (xcex1) of less than 90 degrees.
According to the invention, the substantial length of the cutting edge during a cutting process is substantially constant, thereby resulting in a smooth facing and engraving, which provides a high efficient working. This results in enhancement of the efficiency of the contour line working.
In the indexable type rotary milling tool according to the present invention, a bottom edge serve as a main cutting edge. The main cutting edge ranges from 3 to 35 degrees in cutting edge angle, and is rounded in a radius-round shape. This tool carries out cutting only at a rounded portion in the neighborhood of the tip thereof and hence is shorter in the length of the cutting edge than the conventional button insert. This shortens a substantial length of the cutting edge contacting to the material to be cut in the cutting process for the comer portions and the grooves which are small in the draft of the mold and large in cutting stock, thereby preventing occurrence of the chattering-vibration and thus implementing a stable use of the tool without trouble. The smaller the cutting edge angle of the main cutting edge is, the more smooth the cut face is. If the cutting edge angle is less than 3 degrees, the axial direction-wise depth of cut is reduced, resulting in degradation of the efficiency and obstruction to the practical use. On the other hand, if the cutting edge angle exceeds 35 degrees, the main cutting edge is substantially shortened, resulting in reduction of the lifetime of the insert. More preferably, the cutting edge angle ranges from 10 to 30 degrees.
The main cutting edge preferably ranges from 3 to 7 mm in length. The reason why is that, e.g. in a case where the material to be cut, equivalent to carbon steel S50C is cut at a feed per one tooth (fz) of 1 to 1.2 mm/tooth, if the length is less than 3 mm, the insert is likely to be defected and thus the lifetime thereof is shortened, whereas if the length exceeds 7 mm, the cutting resistance is excessively increased. Therefore, it is preferable that radius of curvature R is increased and that the length is set to 3 to 7 mm.
Generally, it is recommended that the length of the main cutting edge is set to 20 to 60% of the diameter of the inscribed circle in the insert. Moreover, so long as the main cutting edge are rounded at both ends thereof even if it is shaped like a straight line, it has substantially the same characteristic. Therefore, if the main cutting edge is shaped like a straight line and then rounded in proper quantity at both end thereof, the insert is enabled to be formed at a low cost, although the lifetime is slightly degraded.
Moreover, in the indexable insert used for this tool, it is preferable that a line segment extending from an arc-center of the main cutting edge (arc portion) 7 to the connecting point between the main cutting edge 7 and an inner peripheral cutting edge (straight portion) 8 intersects to the inner peripheral cutting edge 8 at an angle of less than 90 degrees (xcex1 in FIGS. 2 and 4). This is because the angle exceeding 90 degrees makes it difficult to set the main cutting edge within the range from 3 to 35 degrees in cutting edge angle and arrange the inner peripheral cutting edge at a suitable angle when the insert is mounted to the holder. Besides, the insert may be an indexable insert which is of a substantially rectangular and two-corner-used type, and shaped like a point-symmetry with respect to the center of the pin hole (See FIG. 2). Further, the insert may be an indexable insert which is of a substantially triangular and three-corner-used type, and has the straight portion 8 and the arc portion 7 (See FIG. 4).
The above and other objects, features, and advantages of the invention will more apparent from the following detailed description taken in:conjunction with the accompanying drawings.